Recently, organic photoconductive materials have been used in various devices, such as electrophotographic photoreceptors (hereinafter, also simply referred to as “photoreceptors”), electrostatic recording devices, sensor materials, or organic light-emitting devices (OLEDs).
An electrophotographic photoreceptor including an organic photoconductive material may be applied not only to photocopiers but also to other various devices, such as printing plates, slide films, micro films, and high-speed printers using laser, light-emitting diodes (LEDs) or cathode ray tubes (CRT) as a light source.
An organic photoreceptor including an organic photoconductive material may improve a film-forming property of a photosensitive layer, may have good flexibility, light weight, and good transparency. Accordingly, the organic photoreceptor may be readily used for designing a photoreceptor having good sensitivity over a wide range of wavelengths through an appropriate sensitization method. Accordingly, there has been an increasing need for an organic photoconductive material instead of an inorganic photoconductive material having a poor film-forming property, poor flexibility, high manufacturing cost, high toxicity, and limitations in manufacturing and handling, and an electrophotographic photoreceptor including the organic photoconductive material.
At the early stage of development, an organic photoreceptor has low sensitivity and durability. However, these drawbacks of the organic photoreceptor have been remarkably alleviated with the development of an electrophotographic photoreceptor including separate materials for specific functions, that is, a material with a charge generation function and a material with a charge transporting function.
The selection ranges for charge generating materials having a charge generation function and charge transporting materials having a charge transporting function for the electrophotographic photoreceptor including separate materials for specific functions are wide. Further, it is relatively easy to prepare an electrophotographic photoreceptor having any of such specific functions.
A variety of materials having strong resistance to light and good charge generation ability have been suggested as charge generating materials of the electrophotographic photoreceptor including separate materials for specific functions. Examples of the charge generating materials are phthalocyanine pigments, squarylium dyes, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine dyes, squaric acid dyes, and pyrylium salt-based dyes.
Various compounds are known as charge transporting materials, such as, pyrazoline compounds as disclosed, for example, in Patent Document 1, hydrazone compounds as disclosed, for example, in Patent Documents 2, 3, and 4, triphenyl amine compounds as disclosed, for example, in Patent Documents 5 and 6, and stilbene compounds as disclosed, for example, in Patent Documents 7 and 8. More recently, pyrene derivatives, naphthalene derivatives, and terphenyl derivatives (such as terphenyl derivatives disclosed in Patent Document 9) that include a condensed polycyclic hydrocarbon system in a center mother nucleus thereof have been developed.
The characteristics required for the charge transporting materials are as follows:
(1) stability against light and heat, (2) stability against ozone, nitrogen oxide (NOx), and nitric acid, which are generated during the charging of a surface of the photoreceptor by corona discharge, (3) high charge transport ability, (4) high compatibility with an organic solvent or binder resin, (5) easy preparation and low costs, and the like.
However, the foregoing charge transporting materials meet only some of these requirements.
Further, the high charge transport ability of the foregoing characteristics requirements is more required than the other abilities. For example, when a charge transporting layer, in which a charge transporting material and a binder resin are dispersed, forms a surface layer of a photoreceptor, the charge transporting material has to have high charge transport ability to ensure a sufficient photoresponsive property.
When a photoreceptor is used in a copying machine or a laser beam printer, a part of a surface layer of the photoreceptor is inevitably scraped by a contact member, such as a cleaning blade or a charging roller. In order to improve the durability of the copying machine or the laser beam printer, a photoreceptor with a surface layer having strong scratch-resistance against the contact members is required.
Accordingly, to improve the durability of the surface layer, the amount of a binder resin in the charge transporting layer, that is, the surface layer, needs to be increased. However, this may deteriorate the photosensitive property of the photoreceptor. This is attributed to a reduction in the charge transporting ability of the charge transporting layer, due to dilution of the charge transport material in the charge transporting layer with the increased amount of the binder resin, in particular, when the charge transporting ability of the charge transporting material itself is poor.
The poor photoresponsive property of the photoreceptor may increase a residual surface potential of the photoreceptor. Repeated uses of the photoreceptor having such a high residual surface potential may not allow sufficient erasure of surface charges from a target exposure portion by exposure to light, and may deteriorate the image quality even at an early use stage. Thus, a charge transporting material having high charge transport ability is required to secure a sufficient photoresponsive property.
With the trends for small and high-speed electrophotographic devices, such as digital copiers and printers, a photoreceptor is more required to have a high sensitive property in order to be compatible with such high-speed processes.
Accordingly, higher charge transport ability is required for the charge transporting material. In particular, in high-speed printing processes, since the time taken from exposure to light to a development process is short, a photoreceptor having a high photoresponsive property is more required. As described above, the photoresponsive property of the photoreceptor depends on the charge transport ability of the charge transporting material. In this regard, a charge transporting material having higher charge transport ability is required.
A variety of compounds having charge mobility which is higher than those of the foregoing charge transporting materials have been suggested as charge transporting materials, for example, as disclosed in Patent Documents 10 to 14.
However, the photoreceptor using an enamine compound as disclosed in Patent Document 10, 11, or 12 does not exhibit satisfactory performance. The compound as disclosed in Patent Document 13 has a symmetric structure due to use of a bisbutadiene-based partial structure, and thus, has poor compatibility with a binder resin and may cause a partial crystallization during formation of a layer.
In order to address these problems, a bulky substituent, such as an aryl group, may be replaced with a small substituent, such as a methyl group; however, the aryl group is advantageous over an alkyl group in terms of electrical characteristics (e.g., hole mobility). Thus, there is a need for further improvement in this regard.
Furthermore, stable sensitivity without a reduction in low temperature environments and high reliability with less change of characteristics in various environments are required for the photoreceptor. However, a charge transporting material satisfying these requirements is not yet available.    [Patent Document 1] Japanese Patent Publication No. S52-004188    [Patent Document 2] Japanese Patent Publication No. S54-150128    [Patent Document 3] Japanese Patent Publication No. S55-042380    [Patent Document 4] Japanese Patent Publication No. S55-052063    [Patent Document 5] Japanese Patent Publication No. S58-032372    [Patent Document 6] Japanese Patent Publication No. H02-190862    [Patent Document 7] Japanese Patent Publication No. S54-151955    [Patent Document 8] Japanese Patent Publication No. S58-198043    [Patent Document 9] Japanese Patent Publication No. H07-048324    [Patent Document 10] Japanese Patent Publication No. H02-051162    [Patent Document 11] Japanese Patent Publication No. H06-043674    [Patent Document 12] Japanese Patent Publication No. H10-069107    [Patent Document 13] Japanese Patent Publication No. H10-239875    [Patent Document 14] Japanese Patent Publication No. 2011-186302